This invention relates to a process for the production of a process oil having an aromatic content of at least 20% by weight and a polycyclic aromatic (PCA) content of less than 3wt %. In this application, aromatic content is measured by the ASTM D 2007 method, and PCA content is measured by the IP 346 method.
Process oils are hydrocarbon mixtures that boil in the same temperature range as lubricant base oils, and like lubricant base oils, are derived from petroleum distillates by solvent extraction. However, unlike lubricant base oils which are rarely employed outside the lubricating field, process oils have a wide range of industrial applications. For example, they are used as solvents, anti-dust compounds, plasticisers and/or extenders for synthetic and natural rubbers, and in the manufacture of printing inks. In order to be suitable for these applications, the compositions of process oils have to be carefully controlled.
Various methods of preparing process oils are known. For example, in GB 1426746, a petroleum distillate is contacted with furfural, and the resulting primary extract is re-extracted with furfural to produce a secondary extract and a pseudo-raffinate. The pseudo-raffinate is distilled from furfural, and then de-waxed and hydrogenated to produce a process oil.
A problem with this process is that it produces a process oil having a low aromatic content as a result of the hydrogenation step. Aromatic compounds, however, impart solvent and swelling properties, which are desirable for a number of applications. The polycyclic aromatic (PCA) content of a process oil, however, should be kept to a minimum, because such compounds are undesirable for health and environmental reasons.
In EP 0417980, a process oil having a high aromatic, and a PCA content of less than 3 weight % is prepared by re-extracting a primary extract under specific temperature and solvent conditions. Specifically, the process is carried out in an extraction column operating at an extract to solvent ratio of 1:1 to 1:1.8, a column head temperature of 50 to 90xc2x0 C., and a bottom temperature of 20 to 60xc2x0 C. The bottom temperature of the column is defined as the lowest temperature in the extraction step. The head temperature of the column is the temperature at the uppermost region of the column. Generally, it is the highest temperature in the column.
We have now discovered an alternative route for the production of process oils. Specifically, the present process involves extracting the primary raffinate under specific operating conditions to produce a process oil having an aromatic content of at least 20% by weight and a PCA content of less than 3 wt %.
FR 1 295 441 describes a process in which a primary raffinate is extracted in a second extraction step. However, the process conditions employed are such that process oils having the composition of the present invention are not produced.
Accordingly, the present invention provides a process for the production of a process oil, the process comprising:
a) contacting a petroleum distillate with a polar solvent in an extraction column which operates with a bottom temperature of 30 to 80xc2x0 C.,
b) withdrawing a primary raffinate from the column of step a),
c) contacting the primary raffinate with a polar solvent in an extraction column which operates at a bottom temperature that is higher than the bottom temperature of the extraction column of step a), and in the range of 60 to 90xc2x0 C.,
d) withdrawing a secondary extract from the column of step c), and
e) removing said polar solvent from said secondary extract.
The process oils produced by the present invention have an aromatic content of more than 20% by weight and a PCA content of less than 3% by weight. Such process oils are suitable for a number of applications, for example, as solvents, anti-dust compounds, plasticisers and/or extenders for synthetic and natural rubbers, and in the manufacture of printing inks.
In step a), the petroleum distillate is contacted with a polar solvent in an extraction column. Without wishing to be bound by any theory, the polar solvent is believed to remove at least some of the PCA compounds originally present in the petroleum distillate. The procedure produces two phases: a primary extract at a lower portion of the column, and a primary raffinate at an upper portion of the column. The primary extract is solvent-rich and comprises naphthenic, and polycyclic aromatic components. This extract is withdrawn from the base of the column. In a preferred embodiment, the polar solvent is removed from the extract and recycled.
Step a) may be carried out using an extraction column which operates at a bottom temperature of 30 to 80xc2x0 C., preferably, 45 to 75xc2x0 C., and especially 55 to 70xc2x0 C. The bottom temperature of the first extraction step (a) is principally dependent on the temperature of the polar solvent and the petroleum distillate but may be controlled by additional cooling at the base of the extraction column. This temperature has an important influence on the selectivity and solvent power of the solvent employed in the present invention, which in turn, affects the final composition of the primary raffinate produced. In one embodiment, a temperature gradient is maintained along the extraction column, such that the head of the column is maintained at 80 to 120xc2x0 C. (head column temperature), whilst the bottom temperature is maintained at 60 to 90xc2x0 C. (base column temperature). The temperature difference between the head temperature and the bottom temperature is from 5 to 70xc2x0 C., preferably 30 to 50xc2x0 C.
The amount of solvent relative to petroleum distillate employed may be in the range of 70 to 300 vol %, preferably, 100 to 250 vol %, and especially, 120 to 170 vol %. It should be recognised, however, that the precise solvent to distillate ratio used will depend on the type of distillate employed. For example, the solvent to distillate ratio may depend on the boiling range of the distillate, and/or its crude origin.
The primary raffinate produced in step a) is withdrawn from an upper portion of the extraction column and re-extracted with a polar solvent in a second solvent extraction step (Step c). The procedure produces two phases: a secondary extract at a lower portion of the column, and a secondary raffinate at an upper portion of the column. In one embodiment, 70 to 100% of the polar solvent is removed from the primary raffinate, before the primary raffinate is treated in the second solvent extraction step. Preferably, 95 to 100% of the polar solvent is removed. The bottom temperature employed in step c) is in the range of 40 to 90xc2x0 C., preferably, 55 to 90xc2x0, and especially 75 to 90xc2x0 C. In one embodiment, step c) is carried out using an extraction column having a head column temperature of 90 to 130xc2x0 C., and a bottom temperature of 70 to 100xc2x0 C. The temperature difference between the head temperature and bottom temperatures is 5 to 70xc2x0 C., preferably 30 to 50xc2x0 C.
The bottom temperature of the second extraction step (c) is dependent on the temperature of the polar solvent and the primary raffinate. Like the bottom temperature of the first extraction step, however, the bottom temperature of the second extraction step may be controlled by additional cooling at the base of the extraction column. The second extraction step is carried out at a higher bottom temperature than step a). This increase in extraction temperature decreases the selectivity of the solvent. When the polar solvent is removed from the secondary extract in step e), a process oil having a PCA concentration of 3 wt % or less is produced. The overall aromatic content of the process oil is more than 20% by weight. The less soluble paraffinic components of the primary raffinate accumulate at an upper portion of the solvent extraction column as a secondary raffinate. This raffinate may be removed from the column, and distilled. The solvent recovered may be recycled for re-use. In one embodiment, the secondary raffinate is purified in distillation column, and then solvent dewaxed to produce a lubricant base oil (e.g. 500N with 96 VI).
Step c) may be performed using a different extraction column to step a). Preferably, however, the same extraction column is employed for both steps.
The amount of solvent relative to petroleum distillate in the second extraction step employed may be in the range of 100 to 400 vol %, preferably, 180 to 300 vol %, and especially, 200 to 250 vol %. As with the first extraction step, the precise solvent to distillate ratio used will depend on the type of distillate employed. For example, the solvent to distillate ratio may depend on the boiling range of the distillate, and/or its crude origin.
Suitable polar solvents for solvent-extraction steps a) and c) include furfural, phenol and N-methylpyrrolidone. Preferably, furfural is employed.